问题 计算题

如图所示的变压器的原线圈1接到220 V的交流电源上.副线圈2的匝数n2=30匝,与一个“11 V, 11 W”的灯泡L连接,L能正常发光.副线圈3的输出电压U3=110 V,与电阻R连接,通过R的电流为0.4 A,求:

(1)副线圈3的匝数n3 .

(2)原线圈1的匝数n1和电流I1.

答案

(1) 300匝(2) 0.25 A

(1)变压比公式对于两个副线圈也适用,则有

,n3=300匝.

2)同理:,n1n2=600匝.理想变压器的输入功率等于输出功率,即P1=P2+P3=11 W+0.4×110 W=55W.原线圈中电流I1=0.25 A

本题考查了变压器以及电功率知识,根据变压器原副线圈匝数关系可求出副线圈3的匝数n3变压器两端功率相等,即P1=P2+P3,列出等式,可求出原线圈中的电流。

选择题
阅读理解

     The trip to the United State opened my eyes to the fact that there are a lot of similarities as well as

differences between American youth and French youth.

     The first difference is in appearance. Going around in the U.S., I found that American youth do not

really care about their appearance. In the morning, they choose something in their closet and wear it with

another thing, often of different colors and styles, without wondering whether their choices make them

look strange. The reason behind this is that they don't care what people look like, but are just interested

in their ideas. I think it's great, but it also causes problems. Since they don't care about their appearance,

they don't really care about their weight. Often they get fat without realizing it.

     There are differences in relationships too. When the American youth fall in love, they don't really think

of the future. Also, there are differences in the rules concerning behavior that is allowed. In some respects, the French are more accepting. For example, in America it is generally unacceptable to make physical

display of love. Lovers, actually, can't kiss or hold each other everywhere they want. People think that not showing love in public places is a way to respect others. On the contrary, the French youth can do almost everything they want.

     As for other types of relationships, there are also big differences. Americans act differently from

French people in front of unknown people. When an American girl, for example, arrives in front of people she has never met before, she will talk with them, trying to create a kind of tie between her and this new

circle of people. The French girls will just think about rather than really do this kind of thing because they

are too shy and lacking in self-confidence.

     To finish, I noticed that in American classes, when pupils want to say or ask something, they just do it.  In general, they don't really care how the others will judge them. In France, it's not the same case. If somebody wants to ask something, he/she will think about it before speaking up. I appreciate the freedom in Americans' behavior, ideas, and ways of expressing themselves.

1. The underlined word "respects" in the third paragraph probably means____.

A. approaches    

B. aspects  

C. honors      

D. means

2. When American youth fall in love with each other, they _________.

A. will not hold each other

B. will rarely display their love in public

C. will always think about their future

D. will kiss wherever they like

3. It can be inferred from the passage that French youth may think more about ______.

A. their relationship's future    

B. their public images

C. their physical displays      

D. their lover's ideas

4. The passage is mainly about the______ between American youth and French youth.

A. differences    

B. friendship    

C. similarities    

D. relationships